Methyl glycolate (HOCH2COOCH3; CAS Registry Number 96-35-3), has been reported to have a variety of uses similar to that of glycolic acid. Methyl glycolate is used as a solvent for semiconductor processes, as a building block for many cosmetics, and as a cleaner for boilers and metals (Ondrey, G., Chemical Engineering, 111(9):20 (2004)). A method to make methyl glycolate from ethylene glycol and methanol has been reported (Ondrey, G., supra). However, this method relies on the use of an expensive gold-based catalyst.
Glycolic acid (HOCH2COOH; CAS Registry Number is 79-14-1) is the simplest member of the α-hydroxy acid family of carboxylic acids. Its properties make it ideal for a broad spectrum of consumer and industrial applications, including use in water well rehabilitation, the leather industry, the oil and gas industry, the laundry and textile industry, and as a component in personal care products like skin creams. Glycolic acid also is a principal ingredient for cleaners in a variety of industries (dairy and food processing equipment cleaners, household and institutional cleaners, industrial cleaners [for transportation equipment, masonry, printed circuit boards, stainless steel boiler and process equipment, cooling tower/heat exchangers], and metals processing [for metal pickling, copper brightening, etching, electroplating, electropolishing]). New technology to commercially produce glycolic acid would be eagerly received by industry.
Enzymatic conversion of glycolonitrile to glycolic acid using an enzyme catalyst (nitrilase or a combination of a nitrile hydratase and an amidase) typically results in the production of an aqueous solution of the ammonium glycolate (U.S. Pat. No. 6,383,786 and U.S. Pat. No. 6,416,980; each herein incorporated by reference). A method to obtain a glycolic acid ester and/or glycolic acid from aqueous solutions comprising ammonium glycolate is needed that separates the desired product easily and efficiently.
One method that has been used to isolate carboxylic acids from the corresponding ammonium salt is reactive solvent extraction. This method has been reported to be useful for extracting lactic acid from ammonium lactate (Wasewar et al., J. Biotechnol., 97:59-68 (2002)). Reactive extraction involves the use of a reactive organic solvent (i.e., an amine) to complex with the acid in the aqueous phase. The first step in the process typically involves acidification of the aqueous solution containing the salt of the desired acid. The acidified aqueous solution is then contacted with an organic solvent typically comprising a reactive amine and one or more diluents. The reactive amine (typically a tertiary alkylamine such as Alamine® 336, Cognis Corp, Cincinnati, Ohio.) reacts with the carboxylic acid forming an acid/amine complex that is soluble in the organic phase. Back extraction is then used to recover the acid from the organic phase. Unfortunately, molar quantities of mineral salts are generated in the process. The economics of using reactive solvent extraction typically requires very efficient organic solvent recycle, as the commercially available tertiary alkyl amines are expensive.
Another method to obtain glycolic acid from ammonium glycolate is thermal decomposition in the presence of an organic solvent optionally including an esterifying agent. The solvent may act by protecting the glycolic acid from reactive ammonia (thereby preventing amide formation) or may act as an organic reactive extraction solvent, thereby aiding in the separation of the acid (Meng et al., US 2004/0210087; hereby incorporated by reference). Optionally, this method can also include an alcohol, thereby creating the ester (which may be more soluble in the organic solvent). The organic solvent may be selected from the group consisting of tertiary alkylamines, alcohols, amides, ethers, ketones, phosphorus esters, phosphine oxides, phosphine sulfides, alkyl sulfides, and combinations thereof. Unfortunately, thermal decomposition in the presence of an organic solvent followed by extraction/back extraction may be problematic as various immiscible fluids form complex physical mixtures that are difficult to separate, a necessary step for effective solvent recycle.
Cockrem (U.S. Pat. No. 6,291,708 B1) teaches rapid heating of a mixture of ammonium salt of an organic acid with alcohol to produce a liquid product stream containing acid, ester, and unreacted ammonium salt. Cockrem fails to address the separation of unreacted salts from the acid and ester.
Filachione et al. (U.S. Pat. No. 2,565,487) teaches a process of producing a carboxylic acid esters from a basic nitrogen salt of an organic carboxylic acid by heating the carboxylic acids in the presence of a refluxing alcohol, typically in the presence of a catalyst such as ammonium sulfate, with an alcohol to produce a complex liquid product mixture comprising the carboxylic acid ester, water, alcohol, and unreacted carboxylic acid salt. The liquid product mixture obtained requires a subsequent distillation step to obtain the carboxylic acid ester.
The problem to be solved is to provide a simple method to obtain a carboxylic acid ester from an aqueous solution comprising the corresponding carboxylic acid ammonium salt.